For pumping liquids and other fluids, gear pumps and the like have experienced substantial acceptance in the art due to their comparatively small size, quiet operation, reliability, and cleanliness of operation with respect to the fluid being pumped. Gear pumps also are advantageous for pumping fluids while keeping the fluids isolated from the external environment. This latter benefit has been further enhanced with the advent of magnetically coupled pump-drive mechanisms that have eliminated leak-prone and unreliable shaft seals, i.e., dynamic hydraulic seals around rotating pump-drive shafts.
Gear pumps have been adapted for use in many applications, including applications requiring extremely accurate delivery of a fluid to a point of use. Such applications include, for example, delivery of liquids in medical and scientific instrumentation. Another application is the delivery of coolant liquids to a location where the coolant liquid can be used for active cooling or temperature control of an object.
With respect to cooling systems, an emerging application of gear pumps and the like is circulated-liquid cooling of microelectronic devices. Particularly demanding aspects of this application include extremely tight spatial constraints for accommodating a liquid cooling system including a pump, extremely high reliability specifications that must be met, minimal cost, and very low energy budget for running the pump.
Ongoing efforts in these and other demanding applications have stimulated interest in development of gear pumps that are smaller, more reliable, less expensive, and more energy-efficient. As gear pumps have been miniaturized to meet these criteria, certain technical challenges have arisen.
One technical challenge pertains to manufacturing the pump-head housing of a light-weight and durable material that is intrinsically low in cost, that can be formed easily and inexpensively, and that holds its dimensions over a long period of time. In this regard, the stainless steel or other metal conventionally used for fabricating larger, conventional pump-head housings has been replaced in many instances with reinforced thermoset plastic. Use of plastic reduces weight substantially and eliminates most if not all the machining steps used in making conventional pump-head housings of metal. The reinforcement (e.g. fibers) provides dimensional and structural stability and durability. Also, compared to metal, plastic is intrinsically lower in cost and advantageously can be molded, which further reduces manufacturing costs.
In addition to use for making miniature pump-head housings, plastic or other suitable is also being used for making the gear set enclosed within a “gear cavity” or “pump-cavity” in the housing of the gear pump-head. Even very small gears made of plastic exhibit high reliability and durability for certain applications. Also, as in conventional magnetically driven gear pump-heads, plastic is used for fabricating the magnet cup of miniature pump-heads. The magnet cup is a sealable enclosure for an axially rotatable magnet that is mechanically coupled to one of the gears (the “driving gear”) and magnetically coupled to a driver (usually configured as a coaxial stator) situated outside the magnet cup. The rotating magnetic field produced by the driver passes through the walls of the magnet cup to the magnet to cause rotation of the magnet. The interior of the magnet cup (including the magnet) is usually bathed by the liquid being pumped by the pump-head, and hence is hydraulically coupled to the gear cavity.
Another technical challenge pertains to tolerance stack-up. As parts of the pump-head are reduced in size, the dimensional tolerances of each part become tighter and more difficult to achieve and control. Also, the tolerances in individual parts “stack-up” as multiple parts are assembled into a pump-head. For example, dimensional tolerances of individual housing parts and rotary members that can be accommodated in a conventional pump-head are intolerable in a miniature pump-head that is five to ten times smaller. Problems with tolerance stack-up arise no matter how the parts are fabricated, whether by molding or machining, and without regard to the particular material from which the parts are fabricated. Also, costs rise substantially in close-tolerance fabrication processes, including molding.
Tolerance issues arise in all the dimensions of miniature parts. For example, a pump-head housing normally comprises at least several housing portions that must be very accurately aligned with each other and with other parts (e.g. the gears and magnet cup) during assembly. Conventional alignment aids include use of alignment pins, mechanical fasteners, or the like, especially if permissible from a cost standpoint. But, with substantial miniaturization of the pump-head, alignment pins become too small to be effective and/or usable in many instances (and the need to hold tight tolerances on the pins themselves makes them prohibitively expensive to manufacture). Hence, there are practical limits to the closeness by which tolerances can be held in miniature parts fabricated by conventional methods and to the tolerance stack-ups that inevitably result when the parts are assembled together. These limits (and the costs associated with overcoming them) must be addressed as miniaturization goals continue to be pursued.
Yet another technical challenge with miniature pump-heads is establishment and maintenance of adequate static seals between housing portions. In conventional larger pump-head housings, O-rings or the like are used to form static seals between mating housing portions. Miniaturization of pump-head housings has required corresponding reductions in the size and thicknesses of O-rings that can be used. This, in turn, raises tolerance problems in molding the O-rings and in forming the glands in which the O-rings are placed for use in forming static seals.
In miniature pump-heads the clearance of the gears or other rotary members relative to the cavity defined in the housing is also a critical issue. For example, gear clearance relative to the housing is directly related to tolerance stack-ups involving the gears as well as the parts of the housing defining the gear cavity. This clearance issue pertains not only to radial clearance of the gears in the gear cavity but also to axial (end) clearance of the gears relative to end walls of the gear cavity. In miniature pump-heads these clearance windows can be tens of microns or less. Excessive clearance (radially and/or axially) can cause the pump-head to exhibit excessive back-flow. “Negative”clearance (i.e., no clearance at all) can result in the gears being bound-up in the housing, which renders the pump-head inoperable. Thus, the difference between too much clearance and insufficient (or even negative) clearance can be extremely small and difficult to control by conventional methods. Since no two identical parts have exactly the same dimensions, due to manufacturing tolerances, and since every component part of a pump-head has its own tolerances, the tolerance stack-up from one pump-head to the next on a manufacturing line can make achieving the right clearance every time nearly impossible when using conventional methods to fabricate miniature pump-heads.